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Improve an AsList method for domains with stored GeneratorsOfDomain #3473

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Jun 5, 2019
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Improve an AsList method for domains with stored GeneratorsOfDomain #3473

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11 changes: 9 additions & 2 deletions lib/domain.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -244,12 +244,19 @@ InstallImmediateMethod( Enumerator,
InstallMethod( AsList,
"for a domain with stored domain generators",
[ IsDomain and HasGeneratorsOfDomain ],
D -> DuplicateFreeList( GeneratorsOfDomain( D ) ) );
function( D )
if HasIsDuplicateFreeList( GeneratorsOfDomain( D ) )
and IsDuplicateFreeList( GeneratorsOfDomain( D ) ) then
return GeneratorsOfDomain( D );
else
return DuplicateFreeList( GeneratorsOfDomain( D ) );
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Thinking about this some more, I am not really sure anymore whether this is an "improvement", as the description of this PR claims: AsList is an attribute. So we are talking about a one-time cost here. Now, with the old code, we'd just call DuplicateFreeList, which performs an O(n^2) algorithm.
With this "improvement", we instead first run IsDuplicateFreeList, which also is O(n^2). And if it fails, we still run DuplicateFreeList. So while there may be scenarios where this saves something (namely memory, but not performance), there are also scenarios where it results in a slow-down.

Are there any real-world applications that motivate this change?

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Oh, I'm completely fine with changing the condition to
HasIsDuplicateFreeList( GeneratorsOfDomain( D ) ) and IsDuplicateFreeList( GeneratorsOfDomain( D ) ).

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@ssiccha ssiccha May 30, 2019 •
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Are there any real-world applications that motivate this change?

I want AsList(Domain([1..3])) to return the range [1..3]. At the moment gap returns the plain list [1,2,3]. As the range [1..3] knows that it is a IsDuplicateFreeList in this situation there should be no reason to copy it and create a new plain list.

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I changed the condition to
HasIsDuplicateFreeList( GeneratorsOfDomain( D ) ) and IsDuplicateFreeList( GeneratorsOfDomain( D ) )

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With HasIsDuplicateFreeList it is of course fine.

fi;
end );

InstallMethod( Enumerator,
"for a domain with stored domain generators",
[ IsDomain and HasGeneratorsOfDomain ],
D -> DuplicateFreeList( GeneratorsOfDomain( D ) ) );
AsList );


#############################################################################
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16 changes: 16 additions & 0 deletions tst/testinstall/domain.tst
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Expand Up @@ -27,5 +27,21 @@ Domain([ 1 .. 5 ])
gap> Domain(FamilyObj(1), []);
Domain([ ])

# AsList and Enumerator for domains which know their GeneratorsOfDomain
gap> r := Immutable([1..3]);;
gap> d := Domain(r);;
gap> IsIdenticalObj(AsList(d), r);
true
gap> IsIdenticalObj(Enumerator(d), r);
true
gap> r := Immutable([1,2,3,1]);;
gap> d := Domain(r);;
gap> IsIdenticalObj(GeneratorsOfDomain(d), r);
true
gap> IsIdenticalObj(AsList(d), r);
false
gap> IsIdenticalObj(Enumerator(d), r);
false

#
gap> STOP_TEST("domain.tst");